Digital Subscriber Link (DSL) service is a network communication protocol. DSL supports fixed bit rates at which packets may be sent over a network. In one common configuration, a customer contracts to receive DSL service from a service provider. On the service provider side, a DSL port connects to a DSL Access Multiplexer (DSLAM), which connects to a router. On the customer side, another DSL port interfaces to a modem that connects to customer premises equipment (CPE). An ATM network connects the service provider-side router and the CPE. Many ports may be aggregated in the network system and connected to the router with a single physical port interface.
For each port there may be many virtual connections. These represent stateful communication setups such as an ATM virtual circuit or Internet TCP connection. At each end of the virtual connection is a software application that can send and receive messages. The messages are carried across the network as packets or frames subdivided into 48-byte ATM cells. The interface in and out of the forwarding device is either 48-byte ATM cells or 64-byte frame segments. Each virtual connection has a quality of service or rate specification. The ATM Forum Traffic Management Specification version 4.1, AF-TM-0121.000, published March, 1999, specifies types of rates, including constant bit rate (CBR), variable bit rate (VBR), and unspecified bit rate (UBR). Variable bit rates can be contracted with a minimum cell rate (MCR), a sustained cell rate (SCR), a peak cell rate (PCR), or a combination of these. Additionally, some VBR virtual connections can be designated real-time (abbreviated as “rt-VBR”), which, among other things, can affect the virtual connections' tolerance of errors or delays in the communication channel. In particular, the tolerance of delay may affect how (or for how long) data for a real-time VBR virtual connection should be queued. Non-real time VBR is abbreviated “nrt-VBR”. A UBR virtual connection can have a priority categorization relative to other UBR traffic. Ports can have peak data rates, describing the maximum rates at which they are capable of transmitting, typically in bits per second. Maximum burst size (MBS) is a parameter specific to a given network protocol and a given implementation. MBS describes the maximum number of cells that may be transmitted continuously from a port over a network link.